1. Field of the Invention
This invention relates to an apparatus for controlling the flow of the exhaust gas in an internal combustion engine with a turbocharger.
2. Description of the Prior Art
In an internal combustion engine having a turbocharger which includes an exhaust gas driven turbine and an air compressor connected to the turbine by means of a turboshaft, there is usually provided an exhaust gas bypass passage which bypasses the turbine and which connects the portions of an exhaust duct on the upstream and downstream sides of the turbine. The bypass passage has therein a waste gate valve to control the amount of the exhaust gas passing therethrough. The waste gate valve is operated by an actuator which operates in response to the delivery pressure of the air compressor. The actuator opens the waste gate valve when the delivery pressure is above a predetermined value, so that almost all or part of the exhaust gas flows through the bypass passage and is discharged into the atmosphere without being fed to the turbine, and closes the waste gate valve when the delivery pressure is below the predetermined value, so that the exhaust gas is fed to the turbine to drive the same.
However, some internal combustion engines also have catalytic converters located in the exhaust duct on the downstream side of the turbine of the turbocharger to purify the exhaust gas. In such cases, since the bypass passage is closed by the waste gate valve immediately after the engine is started and all of the exhaust gas is fed to the turbine, the exhaust gas which is finally fed into the catalytic converter is decreased in temperature, since the turbine has a large heat capacity. Namely, the heat of the exhaust gas is partly lost in the turbine. The decrease of the temperature of the exhaust gas makes it impossible for the temperature of the catalyst to quickly rise, resulting in a poor exhaust gas purification efficiency of the catalytic converter and in a poor emission control.
In order to solve these problems, there has been proposed an actuator, for operating the waste gate valve, which has two pressure chambers separated by a diaphragm, the first pressure chamber being connected to the portion of an intake duct on the downstream side of the air compressor, so that the delivery pressure of the compressor acts on the first pressure chamber, and the second pressure chamber being connected to the portion of the intake duct or carburetor on the downstream side of a throttle valve in the carburetor, so that the intake vacuum acts on the second pressure chamber. The first pressure chamber is adapted to control the waste gate valve in order to prevent the delivery pressure of the air compressor from becoming larger than a predetermined value. The second pressure chamber is adapted to control the waste gate valve especially at a partial load driving condition until the temperature of an engine coolant or the catalyst reaches a predetermined value in order to improve the exhaust gas emission, particularly, immediately after the engine is cold-started.
However, in an actuator having two pressure chambers which, on one hand, controls the amount of the bypassing exhaust gas in response to the delivery pressure of the air compressor to prevent the delivery pressure from being above a predetermined value and which, on the other hand, controls the bypassing exhaust gas in response to the intake vacuum at a partial load driving to improve the emission, the waste gate valve opens, on one hand, when the intake vacuum is superior to an initial spring load W.sub.0 of the waste gate valve and, on the other hand, when the delivery pressure is superior to the initial spring load W.sub.0. That is, a value P.sub.BL of the intake vacuum P.sub.B at which the waste gate valve begins opening and a value P.sub.dL of the delivery pressure P.sub.d of the air compressor at which the waste gate valve begins opening are identical to each other, provided that the directions of action of the intake vacuum and the delivery pressure are opposite to each other. That is, P.sub.dL =-P.sub.BL.
The upper limit of the delivery pressure of the air compressor of the turbocharger is determined taking both the prevention of engine breakage and the requirement of increased engine output into consideration. In most cases, the initial spring load W.sub.0 of the actuator of the waste gate valve is set to be equal to the upper limit of the delivery pressure of the air compressor. That is, W.sub.0 is usually equal to P.sub.dL so as to obtain a maximum engine output. In order to improve the emission control, that is, in order to ensure a quick rise of the temperature of the catalyst especially when the engine is not yet warmed up, it is necessary to widen the active range of the intake vacuum in which the actuator can be actuated, particularly at a partial load of the engine. That is, it is necessary to decrease the minimum value of the intake vacuum necessary for actuating the actuator (for example, from -350 mmHg to -100 mmHg). However, decreasing the minimum value of the intake vacuum necessary for actuating the actuator means decreasing the upper limit (maximum value) of the delivery pressure of the air compressor (for example, from +350 mmHg to +100 mmHg), which, in turn, leads to a decrease of the engine output.